Recently, as a result of rapidly promoted functional capacity and down-sizing of a variety of electronic apparatuses, there is a growing demand for further reduction of the size and weight of electronic parts built therein. In particular, there is a sharply growing demand for flexible printed circuit boards (hereinafter merely referred to as FPC) for mounting a variety of electronic parts against conventional rigid printed circuit boards.
Basically, any of conventional flexible printed circuit boards has such a structure in which "cover-lay film" is coated on the surface of circuit patterns formed on a soft and thin base film. At present, in order to satisfy essential characteristics such as mechanical characteristic, electrical characteristic, chemical resistant property, thermal resistant property, and resistance to environmental condition normally being required for the base film and the cover-lay film, polyimide film has been used most extensively.
Sequential processes for manufacturing such a conventional flexible printed circuit board comprise the following: Initially, a flexible copper-coated laminate (FCCL) is formed by laminating a base polyimide film and a copper foil. Next, a resist pattern is formed on the produced conductor via a screen printing process or a photo-resist process, and then a circuit pattern is formed by executing an etching process. Finally, a cover-lay film is laminated on the surface of circuits of the film having the circuit pattern formed therein. When executing the above sequential processes, either acrylic adhesive agent or epoxy adhesive agent is mainly used for laminating :the base film with copper foil or laminating the cover-lay film on the surface of the built-in circuits.
Nevertheless, any of these conventional adhesive agents proved to be poor in the thermal resistant property and yet contains high rate of hygroscopicity. In consequence, physical properties of the produced flexible printed circuit boards such as the thermal resistant property and dimensional stability are dependent on the physical properties of the used adhesive agent, thus preventing the polyimide film being used for the cover-lay film and the base film from fully exerting own distinct performance characteristic in many cases. In association with the promotion of multiplied layers of flexible printed circuit boards in recent years, there is a growing demand for bi-lateral adhesive sheets having both surfaces of the base film coated with adhesive agent. However, since either acrylic adhesive agent or epoxy adhesive agent is also used for producing such a bilateral adhesive sheet, like the above case, polyimide film cannot fully exert own distinct performance characteristic.
In order to laminate a cover-lay film on the surface of the built in circuits in the course of manufacturing a flexible printed circuit board, it is a conventional practice to execute those sequential processes including an initial step to adhere either of the above adhesive agents onto the surface of a polyimide film, a next step to process a cover-lay film having one-side surface adhered with the above adhesive agent into a predetermined shape, a next step to superpose the formed cover-lay film on the surface of circuits of the polyimide film having circuit patterns formed thereon, a next step to correctly position them, and a final step to thermally bond them via a pressing means. However, when applying either of the above-cited adhesive agents in the course of executing the above processes, since perforation cannot be executed after bonding the cover-lay film with the flexible printed circuit board, before bonding the polyimide film, the conventional art has been obliged to perforate the cover-lay film to provide through-holes or windows at terminals of circuits formed on the conductor or at junctions connected to component parts.
On the other hand, not only difficulty is involved in properly perforating an extremely thin cover-lay film, but working efficiency is also poor and it is costly because the work for positioning the perforated cover-lay film at a predetermined position of the flexible printed circuit board has conventionally and substantially been executed via manual means. Furthermore, if there were too thin thickness of layer of adhesive agent for bonding the cover-lay film, it may lead to the generation of void effect between the flexible printed circuit board and the cover-lay film. Conversely, if there were too thick layer of adhesive agent, it often causes the adhesive agent to ooze itself into perforated domains to result in the faulty conductive effect.
In order to fully solve the above problems, the invention has been achieved by way of providing improved thermoplastic polyimide polymer, improved thermoplastic polyimide film, improved polyimide laminate, and an improved method of manufacturing the polyimide laminate, which respectively feature distinct thermal resistant property and are suitably usable for the formation of cover-lay adhesive agent and layers of adhesive agent for bonding a cover-lay film or flexible copper-coated layers or bilateral adhesive sheets each featuring distinct workability and adhesive property.